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A Plant’s Hidden Half—The Importance of Root Exudates

I spent most of last week at an Understanding Ag class at the Burroughs Farm in Denair, California. Even though it was an almond orchard instead of a pecan orchard, discussions on the six soil health principles and the four ecosystem processes were still applicable. Two of the principles we spent a fair amount of time on were keeping a living root in the soil and enhancing diversity. While we didn’t get too far into the weeds at that meeting, I thought it would be a good idea to dive deeper into the importance of those two principles in a regenerative system.

These two principles are complementary to each other and have a major impact on your soil aggregation and health. The underlying mechanism causing this improvement in our soil is root exudation into the rhizosphere. Whoa! Time out! Exudation? Rhizosphere? Let’s start with a few definitions before we get to the nitty-gritty of the subject. The rhizosphere, first described in 1904 by Lorentz Hiltner (Philippot et al., 2013), generally refers to the portion of the soil found adjacent to the roots of living plants. Plant roots actively regulate the surrounding rhizosphere, influencing the physical, chemical, and biological conditions of this microenvironment by root exudation, creating a dynamic domain for microbe-soil-plant activity and material cycling. Numerous studies demonstrate that species-specific exudates released by plant roots directly impact microbial abundance, and specific metabolites are involved in regulating microbial composition in the rhizosphere.

Plant roots release a wide range of compounds that are generally separated into three categories: low molecular weight solutes (LMW), high molecular weight solutes (HMW), and ions. The LMW component includes carbohydrates (sugars), organic acids, phenolics, amino acids, lipids, flavonoids, and proteins. Mobilization of mineral nutrients in the rhizosphere is primarily accomplished by the organic acids, amino acids, and phenolics, with carbohydrates having only a minor influence on solubility. While the HSW’s most important components are mucilage, cell lysates, and ectoenzymes. Mucilage is secreted from the root cap cells and epidermal cells and has a host of biological functions, including desiccation protection, lubrication for the root as it grows through the soil, ion uptake, enhancing root-to-soil contact, and most importantly, causing aggregation of the soil in the rhizosphere (Marshner, 1995; Ma et al., 2022). Haven’t you ever wondered why soil clings to the roots of a growing plant?

So, let’s look at some of the biological, chemical, and physical factors that impact the quantity and quality of root exudates.

Biological Factors

Plant Species. In a regenerative silvopasture system like a pecan orchard, trees, forbs, and grasses constitute the primary vegetation and have a major impact on the microbial community found in the soil. Different species of plants—or in some cases, different varieties within the same species—have inherent differences in the quality and makeup of their exudates, as well as their exudation patterns. Plants influence the soil microorganism community through biomass production, litter quality, and belowground carbon allocation and nutrient movements. By increasing the plant diversity in the orchard, we can also increase the diversity of the microorganisms in the soil rhizosphere.

Plant Growth Stage. Roots in the initial stages of growth secrete exudates more frequently than older roots. The quantity and quality of root exudates from the same plant varied depending on the plant’s growth stage. The composition was similar for exudates during slow and fast-growing stages, but there were differences in the relative abundance of each compound. A study by Ma et al. (2022) suggested that plants may adjust their exudation patterns over the course of their different growth phases to help tailor microbial recruitment to meet increased nutrient demands during periods demanding faster growth.

Microorganisms. Plants growing in the presence of microorganisms in the rhizosphere can have over 200 percent more exudates produced than plants growing under sterilized conditions. Soil microorganisms utilize root exudates for energy supply and biomass production and, thereby, clearly represent a major source of soil organic carbon (Marschner, 1995). Currently, it is not known if the symbiotic relationship between plant roots and mycorrhizal fungi alters root exudation rates qualitatively or quantitatively. Once mycorrhiza colonized a plant root, a portion of belowground carbon flux will be diverted to the mycorrhiza in exchange for the mycorrhiza delivering nutrients to the plant (Canarini et al., 2019). Despite the widespread presence of mycorrhiza in pecan orchards and other similar tree systems, this relationship has been neglected in root exudate research.

Chemical Factors

Form of Nitrogen. The form of nitrogen present in the soil can influence the composition of the root exudates. Plant roots growing in the presence of ammonium had decreased levels of organic acids in the exudate when compared to plant roots growing in the presence of nitrate. In contrast, plant ammonium nutrition strongly promotes sugar exudation compared to nitrate-grown plants, and switching plants from ammonium to nitrate nutrition in hydroponics reduced sugar efflux by thirtyfold (Canarini et al., 2019).

Nutrient Deficiencies. Soil nutrient deficiencies result in an increase in the low molecular weight solutes in the rhizosphere. The composition of the exudates is also altered depending on the deficient nutrient in the rhizosphere. For instance, potassium deficiency can result in a shift from sugar production to increased organic acid production. In comparison, zinc deficiency in dicots and grasses will often lead to an increase in amino acid, phenolic, and sugar concentrations in the exudate (Marshner, 1995). Organic acids exuded from plant roots can accelerate phosphate dissolution in low-phosphorous soils.

Heavy Metal Toxicities. Pecans growing in highly acidic soils (<5.2 pH) can be exposed to toxic levels of primarily aluminum but sometimes manganese as well. Aluminum inhibits root growth, nutrient uptake, and enzyme activity. Plants respond by releasing increasing levels of organic acids and phenolics, which can form a stable chelate with the aluminum ions, reducing root uptake of aluminum and the associated toxicity symptoms. The mucilage we discussed earlier can also reduce the level of aluminum entering the root cells (Ma et al., 2022).

Physical Factors

Soil. Soil properties is one of the main drivers of the microbial community composition and structure in the rhizosphere. From a plant perspective, we often talk about the seed bank of a soil, which is the natural storage of viable seeds that have persisted in the soil for several years. When favorable conditions return to the soil, you will see plants growing that may not have been seen on that site for decades. The soil can be considered a microbial seed bank since microorganisms can also go dormant and remain viable in the soil for years. When the right physicochemical properties return to the soil, the microbes can become active in the soil rhizosphere again. The location where plants are grown determines which indigenous biota the plant roots are exposed to. 

Drought, Soil Impedance. Drought increases the soil particle’s mechanical pressure (impedance) on the roots and stimulates the root exudates. The increased release of mucilage by the root system allows soil particles to cover the root surface and form a root sheath, which helps to reduce water loss (Marschner, 1995; Ma et al., 2022). The increase in mucilage release also facilitates the transport of metals such as zinc from the soil particles in the mucigel to the root cell. 

Temperature. Sudden increases or decreases in the ambient temperature will stimulate root exudation and the compositional profile of the exudates. This increase may be attributed to a disturbance of the cell membranes due to the temperature variation, resulting in an increase in the diffusion-mediated release of solutes (membrane leakiness) (Hale et al., Ma et al., 2022).

Now that we have reviewed the factors that influence plant root exudation, let’s turn our attention to the importance of exudation on soil structure. Mucilage and polysaccharides usually account for almost 50 to 70 percent of root exudates, but this can vary with plant species. Some of the microorganisms living in the rhizosphere can transform the exudates into exopolysaccharides (EPS), which function as cementing agents between the soil particles. The EPS improve the adherence of the soil to the root and also form microaggregates with the soil and organic matter. This aggregation increases porosity and aeration, and improves water infiltration, retention of water and nutrients, and soil carbon sequestration. Soil microorganisms, especially fungi, can play a major role in the formation of macroaggregates. Fungal hyphae can release glue-like polysaccharide substances that bind microaggregates together, which can then become entangled by mycelium, forming large, stable macroaggregates. Soil organic matter trapped during aggregation will be protected from biological degradation, contributing to long-term carbon storage in the soil.

Now it should be clearer why we want to improve plant diversity in the orchard because this will lead to greater diversity of microbes in the soil. Additionally, I want as much living root as I can grow through the entire year if possible. I am trying to maximize aggregation and improve my soil health each year in the orchard. So, with this new information on the factors affecting plant root exudation, you should be able to utilize it to alter some of your management practices to improve your orchard’s microbial community.

While the industry is still in its infancy, you may have noticed the increase in microbial products to improve plant growth. Among the beneficial microbes, plant growth-promoting rhizobacteria (PGPR) and plant growth-promoting fungi (PGPF) can facilitate a host’s growth and health through various mechanisms, including improving soil structure and nutrient availability, modulating/producing plant hormones, and preventing phytopathogens by direct antibiosis or inducing systemic resistance (Ma et al., 2022). Recently, there has also been an increasing interest in utilizing PGPRs for the biocontrol of soil-borne diseases.

Despite significant progress in understanding the assembly and functions of the rhizosphere microbiome, a huge gap still exists in the understanding of the complex mechanisms of plant-microbe crosstalk in the rhizosphere and the application of beneficial microbiomes for sustainable agriculture, horticulture, and forestry. This new field is not without its challenges, as many biocontrol agents have been effective under experimental conditions but have performed poorly in complex field environments, which is mainly attributed to poor rhizosphere colonization and persistence. Therefore, one future direction is to explore the environmental factors and microbial phenotypes required for colonization and persistence in the rhizosphere environment (Wu et al., 2023). In other words, if we are introducing a foreign microbe to our orchard soil, what are the requirements for it to colonize the existing roots and stay active for several years? Do we already have a microbe in our natural soil biota that simply needs the appropriate plant exudates to flourish? Increasing plant diversity in the orchard may unlock the appropriate habitat.

Along these lines, more in-depth research on characterizing the microbial community that develops with individual cover crops could prove beneficial. We know that different plant species release diverse sets of organic compounds that change rhizosphere conditions and affect microbial community structure, abundance, and activity. Improving our knowledge of how plants differ in their exudation patterns and what different microbial communities they support to promote beneficial interactions in the rhizosphere is one piece of the puzzle. In the future, it may be possible to select cover crops that would support the colonization and persistence of applied PGPR and PGPF products, especially those that have demonstrated an ability to protect against soil-borne diseases.

Literature Cited:
Canarini, A, Kaiser, C. Merchant, A., Richter, A, Wanek, W. 2019. Root exudation of primary metabolites: Mechanisms and their roles in plant responses to environmental stimuli. Frontiers in Plant Science. doi: 10.3389/fpls.2019.00157.
Hale, M.G., Foy, C.L., Shay, F.J. 1971. Factors affecting root exudation. Advances in Agronomy 23:89-109.
Ma, W., Tang, S., Dengzeng, Z., Zhang, D., Zhang, T., Ma, X. 2022. Root exudates contribute to belowground ecosystem hotspots: A review. Front. Microbiol. 13:937940.doi: 10.3389/fmicb.2022.937940
Marschner, H. (1995). Mineral Nutrition of Higher Plants, Second Edn. New York, NY: Academic Press.
Philippot, L., Raaijmakers, J. M., Lemanceau, P., and van der Putten, W. H. (2013). Going back to the roots: The microbial ecology of the rhizosphere. Nat. Rev. Microbiol. 11, 789–799. doi: 10.1038/nrmicro3109
Wu L, Weston LA, Zhu S and Zhou X (2023). Editorial: Rhizosphere interactions: root exudates and the rhizosphere microbiome. Front. Plant Sci. 14:1281010. doi: 10.3389/fpls.2023.1281010
Author Photo

Charlie Graham

Charles J. Graham is the Senior Pecan Specialist at the Noble Research Institute. Noble Research Institute, 2510 Sam Noble Parkway, Ardmore, OK 73401; E-MAIL: